Hi Emma
I would second, big time, Camping Du Letty in Benodet. We first went there on our first overseas camping trip(frame tent) in 1994. We and our kids, then 13,12,10,8 and 6) loved it. We returned every year for a number of years, latterly as caravanners.
The site is large but broken up into hedged "paddocks of 6-8 pitches. Whilst generally quiet and peaceful it has good facilities in terms of a shop, 2 bars and various games. There is no pool but the adjacent beach and "lagoon" is safe for bathing, boating, rock-pooling, etc at all but high tides. We were careful parents but often only saw our kids when they returned for food!! Many campers take boats from cheap inflatables to full-sized motor cruisers which they moor, for free, in the river. We latterly bought a Zodiac inflatable from a French camper and still have it, now well patched, 10 years later.
You can see my review, and 19 other, mainly favourable ones, here.
http://www.ukcampsite.co.uk/sites/reviews.asp?page=4&revid=5461
My review is number 19, ie, the first one in date order and includes a lot of things-do-with-a-family suggestions. We seemed to have set a precedent - pity we're not on commission.
Brittany is an ideal first-time destination for families. We travelled Brittany Ferries Plymouth to Roscoff which meant only an hour or so's pleasant, rural drive to Benodet. That route has, however, become rather expensive nowadays. It is possible to book Du Letty and the Brittany Ferry as a package from the Camping and Caravan Club but we always found it cheaper to just book the ferry with them and the site directly - in fact the first year and a number of others we got good pitches at Du Letty without booking.
If you do go to Du Letty ask for a pitch on the edge of the beach. One year we managed to move to one overlooking the beach with only the low hedge in between - a superb spot.
Brittany is a super place for family holidays and I wish you a happy time. It brings back so many memories of carefree, mainly sunny, hols just writing this.
Regards
Tim